1. Field of the Invention
The present invention relates to a detecting device capable of detecting the absolute position of a numerical control machine tool, and more particularly to an electronic multi-shaft absolute position detecting device.
2. Description of the Related Art
The prior manually operated machine tool had been replaced by the current computerized numerical control machine tool, which has the following advantages: easy change of processing manner, reducing requirement for labor, reducing processing time and possible error, high reliability, good efficiency, etc.
However, in the computerized numerical control machine tool, because the signals produced by the encoder or linear scale are increment position signals, only the relative position of the machine is provided. As a consequence, when initiating the machine or when the power is restored after power is cut off, the absolute position of the machine can be obtained only by means of returning to home position. However, in consideration of the safety of the operator and machine, most machine tools are unable to automatically execute the operation of returning to home position. This operation must be done by the operator. This shortcoming limits the application of the computerized numerical machine tool in unmanned automatic processing system.
Although the Gray Code Encoder, which is capable of detecting the absolute position signal, can be disposed on the rotary shaft for detecting the absolute position signal so that the computerized numerical control machine tool can automatically recover when the power is restored to continuously execute the processing operation, the Grey Code Encoder has the following disadvantages:
1. The Gray Code Encoder is a binary encoder and thus it can only provide binary analysis (such as 256,1024,4096, etc.), which is different from the decimal system used in computerized numerical control machine tool. This often causes troubles in processing.
2. According to the design principle of the Gray Code Encoder, to increase an analysis level by one bit, a circle of photo-screen and a set of photodiodes need be added thus increasing difficulty in manufacturing. Therefore, high analysis level (such as over 10,000/ev) is rarely achieved to satisfy the high accuracy requirement of the computerized numerical control machine tool.
3. The Gray code Encode can only provide the absolute angle of single rotation. As to the computerized numerical control machine tool with more than one rotation, additional gears, etc. must be installed so as to provide the detection of the absolute position signals of the entire distance of travel. The cost is therefore increased.
4. The tooth clearance, back clearance and screw rod manufacturing error causes location error. Thus, in a sophisticated machine tool, a linear scale is used to overcome the location error. Therefore, the Gray Code Encoder can not be used.
As evident from the above, the Gray Code Encoder obviously is not an ideal means for detecting the absolute position signals.